No local knowledge but my advice is, once you order it, be there when the load is delivered and get up on the lorry and look at it before you let them tip it. Look for rubble, couch grass roots and other perennial weed roots, lumps of clay etc which may or may not be hidden beneath the surface so use a fork or spade to check it. Once it's tipped, it's tipped. Whatever you say, they're not going to shovel it back on and take it away...

No-one has a consistent supply of topsoil - it's whatever has just been removed prior to new building work - so it's always variable. Worth asking for info on the site it's just been removed from (farmland, demolition area etc) too.